(1) Field of the Invention
The present invention relates to a constant flow valve used in liquid transport piping to ensure a constant flow rate for a domestic water supply, industrial water, water for irrigation, petroleum supply, and chemical liquids supply, or the like.
(2) Description of the Related Art
As automatic adjustable valves for a constant flow rate control, a constant flow valve having a constant flow rate control function and a preset flow rate change function, and a constant flow valve having a constant flow rate control function and an opening and shutting function are well known. The former is disclosed as a "Flow Constant Automatic Control Valve" in Japanese Examined Patent Publication (Kokoku) No. 57-57729, and the latter is disclosed as a "Fluid Valve" in Japanese Unexamined Patent Publication (Kokai) No. 59-231274.
The "Flow Constant Automatic Control Valve" disclosed in No. 57-57729 is provided with a narrowing flow portion in a flow passage of a valve casing, maintains a constant pressure differential upstream and downstream of the narrowing flow portion through a diaphragm type pressure differential adjusting valve operating in response to a detection of a pressure differential to control a flow rate to a constant value, and adjusts the section area of the narrowing flow portion by an umbrella-shaped adjusting device, thus enabling a change in a preset flow rate. However, this type, and other well known types of valves having a diaphragm type pressure differential adjusting valve, as well as throttle valves, must be provided with stop valves when an emergency valve closure is required, because the construction of these valves does not include an opening and shutting mechanism.
Also, although the "Fluid Valve" disclosed in No. 59-231274 can carry out a constant flow rate control and an opening and shutting operation, it can not change a preset flow rate.
Thus, in the prior art, a constant flow valve having three functions, i.e., a constant flow rate control function, a preset flow rate changing function, and in opening and shutting function, and having a simple and compact construction, is not known.